The level of sun protection provided by a fabric is affected by several factors, with the most important being the weave, followed by color, weight, stretch, and wetness. A denser or less open weave, as well as a heavier and less stretchy fabric, provide better protection. However, getting a fabric wet can significantly reduce its protection, except in the case of silk and viscose which become more protective when wet. Polyester, which contains a UV-absorbing benzene ring, and fabrics that have UV absorbers added during manufacturing, also offer higher protection levels.

UPF (Ultraviolet Protection Factor) is a relatively recent rating system for sun protective textiles and clothing. It measures the ratio of UV that causes sunburn, which is blocked by the fabric. For example, a fabric with a UPF rating of 30 blocks 96.7% of UV, allowing only 1 unit of UV to pass through for every 30 units that fall on the fabric. This is in contrast to the traditional SPF (Sun Protection Factor) measurement, which is based on human sunburn testing. UPF measurements are made using a laboratory instrument.

Standard for Sun Protective Clothing and Swimwear: